| Summary: | Observations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales Observations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales. Basal shear stresses in a well-defined zone north of the center line (orographic left) were approximately 7% and 16% lower in spring and summer, respectively, than in winter. Correspondingly higher stresses were found near the margins. These changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1.2 and 1.5 times larger in spring and summer than in winter. These results were inferred with a simple inverse finite-element flow model that can successfully reproduce bulk surface velocities and tiltmeter data. Stress redistribution between the well-defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions (ice–bed decoupling or enhanced till deformation), thereby causing large variations in surface velocity and strongly influencing the glacier’s net motion during summer. This project was supported by grants OPP-0115819 and OPP-0414128 of the US National Science Foundation. The fieldwork could not have been completed without the help of A. Arendt, A. Behar, J. Brown, A. Bucki, S. Campbell, T. Clarke, L. Cox, K. Echelmeyer, D. Elsberg, W. Harrison, U. Korotkova, A. Mahoney, D. Moudry, M. Parrish, D. Pomraning, B. Valentine, R. Woodard and S. Zirnheld. C. Larsen provided important, last-minute assistance with instrument assembly. Logistics support was by Veco Polar Resources, Tundra Helicopters and Ultima Thule Air Service. Discussions with W. Harrison, K. Echelmeyer, R. Motyka and A. Arendt improved the manuscript. We would also like to thank the scientific editor, J. Walder, and J. Kavanaugh and D. Cohen for insightful reviews. Yes
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